Add ostruct as bundled gem

See https://bugs.ruby-lang.org/issues/20309

Author: numbata

Gemfile

@@ -19,6 +19,7 @@ group :development, :test do
   gem 'grape-entity'
   gem 'pry', platforms: [:mri]
   gem 'pry-byebug', platforms: [:mri]
+  gem 'ostruct'
   gem 'super_diff', require: false
 
   grape_version = ENV.fetch('GRAPE_VERSION', '2.2.0')